Heretofore it has been difficult and time-consuming to repair or replace card-on-board units in the field. For example, in the Monolithic System Technology card-on-board package used in the IBM 370 System, which is described in the IBM Maintenance Library Manual SY22-6739-5, Copyright 1969, the printed circuit board and printed circuit cards could not be removed as an entire unit. The cards had to be removed first from the card guidance system, then the card guidance system dis-assembled and removed from the board assembly. As the cards were removed, they had to be placed in the card container in proper sequence to maintain the sequence when re-plugging into a new board assembly. To remove the board, it was necessary to first remove the cards and then disassemble the guide frame from the board assembly. A multitude of screws were involved some of which oftentimes would fall into the unit. Additionally, a mass of I/O (input-output) wires had to be individually disconnected. It would take from 8-10 hours to remove and re-install a board in its supporting structure.
To actually repair or change the board in the field involved a tedious time-consuming task. Overflow and engineering change wires were either wire wrapped or clipped to the back of the board. To make a change or repair, it was generally necessary to remove a plurality of these overflow and/or engineering change wires and, in addition, they had to be individually tagged so that they could be reinstalled into their proper position. If the field engineer could not repair the board, it would have to be removed and sent back to the factory. In either event, the customer was faced with an undesirable excessive amount of machine downtime.
Also, present day technology has dictated that the overflow and engineering change wiring be permanently bonded to the circuit boards to improve electrical performance. Due to the small wires and density now used, engineering changes cannot be made in the field without the use of sophisticated equipment to perform deletions and wire additions resulting in an expensive operation. It became evident that an improved card-on-board package assembly was needed which could be quickly removed and replaced in the field.